The quality of the bore, i.e. smoothness without machining marks on the rifling and concentricity of the bore matters greatly. Actually, no such thing as hard cast bullets. You want some degree of softness so bore will be sealed. I've found that bullets cast of wheel weights with 1% tin added to be best with perhaps a .25% of Lino added. This can be pushed to 1800-1900 with little to no leading. Add copper or aluminum gaschecks and they can be pushed to 2400. Yes proper sizing is very important.
